We studied the role of CD28/CTLA4 costimulatory T-cell activation pathway on the pathogenesis of MRL/lpr mice. Administration of CTLA4IgG from day 0 significantly inhibited autoantibody production such as anti-double-stranded DNA antibody and rheumatoid factor. In addition, end-organ diseases in kidney, salivary gland, and liver were significantly improved. Improvement of pathologic findings coincided with a significant improvement in survival. At 350 days of age, 90% of mice treated with CTLA4IgG from day 0 were still alive, compared with none of mice treated with hIgG. As expected, activation of conventional T cells was significantly inhibited after CTLA4IgG treatment. However, lung disease that was characterized by perivascular accumulation and interstitial infiltration of lymphocytes and macrophages was not inhibited. Even after CTLA4IgG treatment from day 0, pathologic findings of lung disease were not improved. Additionally, the expression of activation markers such as B7-1, B7-2, CD71, ICAM1, and LFA1 on Mac1+ fraction in both spleen and lung and the concentration of TNF[alpha] in bronchoalveolar lavage fluid were not significantly suppressed. These results demonstrated that lung disease was independent of the CD28/CTLA4-B7 pathway. Thus, this study emphasizes the differential dependence of the CD28/CTLA4-B7 pathway in development of diseases in MRL/lpr mice. (Lab Invest 1999, 79:317-326).